A corresponding solution is known from published international application, WO 2004/013585 A1. Applied in automation technology, especially in process automation-technology, are field devices, which serve for determining and monitoring process variables. Examples of such field devices are fill level measuring devices, flow measuring devices, analytical measuring devices, pressure and temperature measuring devices, moisture and conductivity measuring devices, density and viscosity measuring devices. The sensors of these field devices register the corresponding process variables, e.g. fill level, flow, pH-value, substance concentration, pressure, temperature, moisture, conductivity, density and viscosity.
The terminology ‘field devices’ includes in connection with the invention, however, also actuators, e.g. valves or pumps, via which, for example, the flow of liquid in a pipeline or the fill level in a container is changeable. A large number of such field devices are manufactured and sold by members of the firm, Endress+Hauser.
As a rule, field devices in modern automation technology plants are connected via communication networks, such as HART-multidrop, point to point connection, Profibus, Foundation Fieldbus, with a superordinated unit, for instance a control system or control room. This superordinated unit serves for process control, for process visualizing, for process monitoring as well as for start-up and for servicing the field devices. For the operation of fieldbus systems, necessary supplemental components, which are connected directly to a fieldbus and which serve especially for communication with the superordinated units, are likewise frequently referred to as field devices. These supplemental components include e.g. remote I/Os, gateways, linking devices, controllers or wireless adapters.
Depending on application, the field devices must satisfy the most varied of safety requirements. In order to meet the respective safety requirements, e.g. the IEC61508 safety integrity level (SIL standard), the field devices must be redundantly and/or diversely designed.
Redundancy means increased safety through doubled or multiple design of all safety relevant, hardware and software components. Diversity means that the hardware components located in the different measurement channels, such as e.g. a microprocessor, be from different manufacturers and/or be of different type. In the case of software components, diversity requires that the software stored in the microprocessors originate from different sources, e.g from different manufacturers, respectively programmers. Via all these measures, it should be assured that a safety critical failure of the field device as well as the occurrence of simultaneously arising systematic failures in the providing of measured values be excluded with high probability.
An example of a safety-relevant application is fill level-monitoring in a tank, in which a flammable or even a nonflammable but water-endangering liquid is stored. In such case, it must be assured that the supply of liquid to the tank is immediately interrupted, as soon as a maximum permitted fill level is achieved. This, in turn, assumes that the measuring device highly reliably detects fill level and works faultlessly.
While in the case of known solutions the measurement channel is redundantly and/or diversely designed, nevertheless, the voter, usually a microprocessor, represents the Achilles' heel of a field device, which should satisfy high and highest safety requirements. The microprocessor is monolithically embodied. If there is here a dangerous failure (corresponding to the nomenclature of the above mentioned standard), then the field device fails. In order to fulfill the requirements of SIL 3, the fraction of dangerous failures to the total of all possible failures must lie at a maximum of one percent. With a conventional microprocessor, this safety level cannot be achieved.
In order to solve this problem, in the non-prepublished German application, DE 10 2012 106 652.3, filed on Jul. 23, 2012, a field device is described, whose voter is embodied as a majority voter and includes three stages:                a comparator stage, which compares the output signals delivered by the individual measurement channels with one another;        a failure recognition stage, which detects by suitable combining of the output signals of the comparator stage a failure occurring in a measurement channel, and        an output selection stage.        
The content of the German application, DE 10 2012 106 652.3 is incorporated here by reference, especially as regards the voter.
Still, no field device is known, which fulfills the high safety level also in the region of the electrical current output module e.g. in the case of a 4-20 mA two or four wire, field device.